Indoleacetic acid Auxin

Scheme 1. Molecular structure of the plant growth hormone auxin (indoleacetic acid, IAA). Extremely small amounts (nanomolar) can be detected by the auxin standard test 5 mm long segments of pea shoots elongate faster in the presence of exogenous auxin, which can be taken as a sensitive assay...

Higher plant studies of tryptophan metabolism have focused on its role as a precursor for indole alkaloids or as a precursor of the auxin indoleacetic acid. The conversion of tryptophan to indoleacetate has been the subject of many investigations for many years. There is considerable uncertainty over the route of degradation. The following diagram presents the possible pathways which have been suggested most frequently ... 

Indolebutyric acid org chem C12H13O2N A crystalline acid similar to indoleacetic acid in auxin activity. Abbreviated IBA. in,dol-byu tir-ik as-od indoxyl orgchem (C8H6N)0H A yellow crystalline glycoside, used as an intermediate in the manufacture of indigo. in dak-sol ... 

The most promising leads have come from studying the hormonal requirements of the system. By themselves, we can find no indication whatsoever of any growth promotion by any of the alkyl lipides. When gibberellic acid is also applied, response remains small. If indoleacetic acid, or another auxin, is added, the sections show the classic bioassay response—and this is further stimulated by the active lipides. In the presence of both auxin and gibberellic acid, however, the sections show their greatest capacity to elongate still further in the presence of the alkyl lipides (I). Remarkably, the lipides are most effective at concentrations corn-... 

Figure 8.5. Photographs of Daucus carota cell suspensions treated with humic substances or hormones. Indoleacetic acid (IAA) 1-naphthylacetic acid (NAA) HEf (humic matter fraction <3500Da). (A, control + IAA B, D, control + HEf C, control + NAA). Reprinted from Muscolo, A., Bovalo, F., Gionfriddo, K, and Nardi, S. (1999). Earthworm humic matter produces auxin-like effects on Daucus carota cell growth and nitrate metabolism. Soil Biol. Biochem. 31,1303-1311, with permission from Elsevier Limited.

Plant-inhibitory effects of some AB are auxin-mediated, illustrated by direct uptake of bacterially produced indoleacetic acid (IAA). Plant response to microbially... 

Results with plants show the azalog of indoleacetic acid to be a powerful auxin in Pisum (pea) and Avena (oat) growth.This and the propionic acid isomer also proved to be effective with Partheno-cissus tricuspidata (Boston ivy) tissue. 7-Azatryptophan was found to inhibit a-amylase synthesis in barley endosperm tissue. 

LXXXIV) or undergo further oxidation and base-catalyzed decarboxylation (LXXXVI) to the labile nonisolable 3-methyleneoxindole (LXXXVIIa), characterizable as the sodium bisulfate addition product (LXXXVIIb). 8-Methylene oxindole (LXXXVIIa) bears a spectral resemblance to the oxidation product of auxin produced by peroxidase or indoleacetic acid oxidase from the fungus Omphalia ftavida (Ray and Thimann, 1956). 

The plant hormone auxin has been shown to be radiosensitive. The product of the irradiation of auxin (p-indoleacetic acid) is a polymer similar to that obtained in the radiolysis of indole. 

An in vitro bioassay was then performed to measure the auxin-like properties of these compounds, based on a procedure from Cleon Ross(14)(Fig. I). This assay is based in principle on the growth response to auxins of stem segments of Pisum sativum. Measurement of segment weight and transectional area was compared to the untreated control.In this manner, a response curve was obtained for each compound. Indoleacetic acid (IAA) gave a typical response curve, as shown AC 78,299 gave an auxin-like response while AC 78,167 was inactive(Fig. 2). 

An auxin in apple endosperm (565) not identical with indoleacetic acid was identified as ethyl indoleacetate (863, cf. 697), but this may be an artifact due to esterification during isolation (377). 

Other indolic auxins besides indoleacetic acid occur in plants (e.g., 396). Both indoleacetaldehyde (51, 107) and indoleacetonitrile (51, 377, 452) have been claimed to be plant growth hormones. Indoleacetonitrile has been isolated from plant sources (377) and shown to be of wide distribution, but whether it is a hormone per se or only acts as a precursor of indoleacetic acid is not yet clear (cf. 838, 865). 

Inhibition of Limonoid Biosynthesis by Auxins. Auxins are potent inhibitors of nomilin biosynthesis in citrus seedlings (31). For instance, up to 91% inhibition was observed when 10 ppm of indoleacetic acid was fed to the stem of a lemeon seedling two days prior to and two days following feeding of 25 pCi of 14-C acetate (Table 1). Other auxins tested include 1-naphthaleneacetic acid (NAA), indolepropionic acid, indolebutyric acid, 3-indole acetonitrile, ethyl indole-3-acetate, 3-indoleacrylic acid, 3-(2-hydroxyethyl)indole, indole-2-carboxylic acid and 2,3,4-trichlorophenoxyacetic acid. They were all very effective. 

There are three major classes of synthetic auxins the aryl acetic acids, which include indoleacetic acid (lAA) itself (Fig. 1 structure 2-1) and 1-naphthyl acetic acid (2-4), phenoxy acetic acids, represented by 2,4-dichlorophenoxy acetic acid (2,4-D) (2-7) and the benzoic acids, e.g. 2,3,6-trichloro benzoic acid (2-9). It remains the case however, that there is no stmcture-activity proposal which satisfactorily covers all molecules which are known to have auxin activity. As is well known, auxins have multiple effects, including promotion of cell elongation, cell division and gene expression. There may well be different receptors for each effect, and differences between receptors. An auxin binding protein (ABP), has now been characterised, and is possibly a receptor controlling cell elongation [3]. 

A number of factors have been studied for their influence on nicotine production. Of these the negative effect of auxins, and in particular 2,4-D, on alkaloid production is worth mentioning (202-204,211,220, 225,226,229,255). In root cultures the addition of indoleacetic acid (lAA) also reduces alkaloid production (196). Light was reported to inhibit nicotine formation (50,255). In a green cell suspension, however, increased nicotine levels were found on illumination (229). Ikemeyer and Barz (243) reported that a photoautotrophic cell line of N. tabacum did not produce nicotine, whereas a heterotrophic cell line did accumulate this alkaloid. Elicitation with a preparation of the fungus Phytophthora megasperma did not affect the nicotine levels of these cell lines. Addition of organic acids to the medium resulted in increased alkaloid formation in callus cultures (up to 3.25%) (230). For a review of the various cultural factors which influence secondary metabolism, the reader is referred to Mantell and Smith (255). 

Auxin, see also Indoleacetic acid callus formation and, VI, 239 8-Azaguanine, see under Guanazolo Azotobacter chroococcum, niacin content of, VI, 184, 185... 

Indoleacetic acid, see also under Auxins effect of combined application of thiamine and, on roots, VI, 259ff. as growth factor for callus tissue, VI, 239... 

It has been reported thata-naphthaleneacetic acid (NAA) or indoleacetic acid can induce the expression of acidic LOXs in cultured immature soybean cotyledons [32]. Further studies showed that one of NAA-induced LOXs is LOX-4 (C. Wang and D.F. Hildebrand, unpublished work). LOX-4 is expressed in germinable somatic embryos, but not in non-germinable ones [33]. The fact that exogenous auxin can induce LOX-4 expression in cultured soybean cotyledons ([32], C. Wang and D.F. Hildebrand, unpublished work) supports the hypothesis that auxin may play a role in induction of seedling LOX expression in germinating soybeans. 

L-Tryptophan derivatives 3-Indoleacetic acid (D 21) and related compounds, e.g., indoleacetoni-trile (D 9.4) Plants (Apical meristems of stems and roots) Regulation of various growth and developmental processes (auxin activity)... 

The demonstration that the growth of plants is under hormonal control has profoundly influenced the direction of botanical research. Among the well-established plant hormones the most studied belong to the group known as auxins. At present, all conclusively identified natural auxins are compounds chemically related to indoleacetic acid (X). 

Investigations of indolic auxin metabolism in plants have centered about attempts to understand plant growth regulation. As a consequence, factors which limit the concentration of indoleacetic acid in plant tissue have received the most attention. These include its S3mthesis, its catabolism, and its removal from the site of action by other means. This will also provide a convenient division for discussion here. Full reference to the original work has been provided elsewhere. ... 

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